"I think this just feels like it's time to end it," the 67-year-old La Russa said at a news conference at Busch Stadium.
The World Series win over Texas was the third of La Russa's 33-year career. The manager guided the Cardinals to the championship despite being 10 1/2 games behind Atlanta on Aug. 25 for the final playoff spot in the National League.
La Russa retires third on the all-time wins list, 35 behind second-place John McGraw. In addition to this season, he won titles in Oakland in 1989 and St. Louis in 2006. He is the first manager to retire immediately after his club won the World Series, according to STATS LLC.
"Other than some of the personal attachments, I feel good," La Russa said. "I feel good that this is the right decision."
La Russa said there wasn't a single factor that led to his decision, but he began having doubts about returning for 2012 midway through the season. In late August he told general manager John Mozeliak and other team officials.
La Russa said the timing of those discussions - about the time the Cardinals appeared to be out of wild card contention before their miraculous run - was pure coincidence. He said he simply felt it was time to go, a feeling that didn't change even as the Cardinals squeaked into the playoffs on the final day of the season, then upset the Phillies, Brewers and Rangers.
He spoke with little emotion at the news conference with one exception, when he paused to compose himself as he thanked his wife, Elaine, and two daughters for putting up without him over much of the past 33 years. But he did say his meeting with players after Sunday's parade and celebration was short but emotional.
"Some grown men cried," La Russa said, then he joked, "I kind of liked that because they made me cry a few times."
Mozeliak said work is under way to find a new manager for the first time since La Russa was hired prior to the 1996 season. A search committee will be formed. Mozeliak did not speculate on how long the process might take.
La Russa answered flatly, "No," when asked if he'll ever manage again. He also said he had no plans to be a general manager, but said he is open to some sort of baseball job in the future.
"Maybe open a book store," he said.
Chris Carpenter, who won four times in the postseason, including the decisive Game 7, said La Russa gathered the team together in the weight room moments after Sunday's celebration at the stadium, along with Mozeliak and principal owner Bill DeWitt Jr..
He spoke about how proud he was of the team's championship run, "and then he said that he was done," Carpenter said. "Everybody was surprised, shocked. I think every single guy in there was emotional and gave big old hugs on the way out."
Carpenter said the behind-the-scenes La Russa is different than the public persona - including a great sense of humor. But he lauded La Russa for always having his team play at its highest possible level.
"I'm not sure there are a lot of people that can match the preparation, the dedication and the ability to put it all together," he said.
Mozeliak said the team will have a "long list" of candidates for a job that will likely be considered among the best in baseball given the strong returning team - whether or not Albert Pujols decides to come back - and based on the strong fan support in St. Louis.
"There's going to be a lot of names that we'll consider," Mozeliak said. "We want to do our due diligence. We want to be smart."
DeWitt said replacing La Russa will be a tall task.
"We're not going to find a Tony La Russa out there, given his career and what he's accomplished, what he's meant to the Cardinals," DeWitt said. "We're in a pretty good situation for the future. But it'll be different, no question about it."
La Russa's decision leaves the future of his coaching staff up in the air. Mozeliak said the new manager will be given autonomy to hire his own staff or retain some or all of La Russa's. Asked about pitching coach Dave Duncan, La Russa's longtime right-hand man, Mozeliak did note that Duncan is under contract for 2012.
As for Pujols, Mozeliak noted that he has a strong relationship with the only manager he's ever played for, but doubted it would be a factor in whether the free agent first baseman stays.
"He probably understood that Tony is not going to manage forever," Mozeliak said.
La Russa was a .199 hitter in a brief major league career. He began as a manager with the Chicago White Sox in 1979. He guided the Oakland A's to three straight American League pennants in 1988-1990 and the 1989 World Series title over the Giants.
La Russa was hired by the Cardinals in October 1995, soon after the new ownership group purchased the team from Anheuser-Busch. His impact was immediate - the Cardinals won the NL Central and came within a game of going to the World Series in 1996, losing to the Atlanta Braves.
Overall, St. Louis went to the playoffs nine times in La Russa's 16 seasons, won pennants in 2004, 2006 and this year, and won two championships, over Detroit in 2006 and this season, rallying to win the final two games over Texas, including the memorable Game 6 when the Cardinals trailed five times and were down to their last strike in two innings. His teams were successful on the field and in the stands - the Cardinals drew 3 million fans in 13 of La Russa's 16 seasons.
La Russa, who won 2,728 regular-season games, including 1,408 with the Cardinals, said he never considered coming back simply to reach No. 2 on the all-time wins list.
"I'm aware of the history of the game, but I would not be happy with myself if the reason I came back was to move up one spot," La Russa said.
Washington Nationals manager Davey Johnson said La Russa picked the right time to leave.
"I tip my hat to him. He's had a great career. What a way to go out," said Johnson, who at 68 is a year older than La Russa. "If you're going to retire, that's the way to go out; a world champion."
AP Sports Writer Howard Fendrich in Washington D.C. contributed to this report.